In a mobile communication system, the number of base stations, or the number of cells or sectors, to which a mobile station apparatus such as a user terminal is linked via radio at one time constantly changes according to the current location or radiowave reception conditions of the mobile station apparatus or as the mobile station apparatus moves from one location to another.
Adding a new communication cell or deleting a communication cell is performed by a radio network controller (RNC) connected to the base station apparatus. For this purpose, the RNC sends a signal to notify the base station apparatus and mobile station apparatus of a change in the number of communication cells, and receives information responding to the change in the number of communication cells from the base station apparatus and mobile station apparatus.
When the number of communication cells is changed for a particular mobile station apparatus, if the total amount of power received at that mobile station apparatus changes, the mobile station apparatus may not be able to properly reproduce the received signal. In view of this, the base station apparatus connected via radio to the particular mobile station apparatus adjusts the transmit power of each cell so that the total amount of the transmit powers of all the serving cells is maintained constant during communication in order to prevent the received power at the mobile station apparatus from being affected by the change in the number of communication cells. For example, suppose that the number of communication cells has increased from 1 to 2; then, the base station apparatus reduces the transmit power of each cell to one-half its original transmit power.
In a mobile communication system, the base station apparatus maintains a radio link with each mobile station apparatus by utilizing a signal received from the mobile station apparatus. For example, if the base station apparatus utilizes the call control defined in the Release 99 standard developed by the 3GPP (Third Generation Partnership Project), the base station apparatus controls the power of transmit signals. Such signals are transmitted, for example, via a downlink-dedicated physical data channel (DL-DPDCH) and a downlink-dedicated physical control channel (DL-DPCCH). The base station apparatus controls the transmit signal power by utilizing a transmission power control command (TPC) received from the mobile station apparatus via an uplink-dedicated physical control channel (UL-DPCCH).
Further, in the call control provision of the High-Speed Downlink Packet Access (HSDPA) defined in the Release 5 and later standards developed by the 3GPP, High-Speed Physical Downlink Shared Channel (HS-PDSCH) is defined as one of channels for carrying user data. Further, in the HSDPA, High-Speed Shared Control Channel (HS-SCCH) is defined that carries control information for transmitting data over the HS-PDSCH. The transmit power of the HS-SCCH is determined by the sum of the transmit power of the DL-DPCCH and the transmit power offset of the HS-SCCH. In a mobile communication system employing the call control conforming to the HSDPA, when the number of communication cells is changed for a mobile station apparatus, the transmit power of the DL-DPCCH increases or decreases correspondingly. Accordingly, the base station apparatus adjusts the transmit power offset of the HS-SCCH so that the transmit power of the HS-SCCH will not be affected by the change in the number of communication cells. To adjust the transmit power offset of the HS-SCCH so as to accommodate the change in the number of communication cells, control signals are transferred between the RNC, the base station apparatus, and the mobile station apparatus in a prescribed order.
As described above, in order to maintain the communication, the mobile station apparatus and the base station apparatus share the necessary control information and update the setting at predefined timing based on the control information. In view of this, a technology has been developed that allows the mobile station apparatus and the base station apparatus to share prescribed control information so that the mobile station apparatus and the base station apparatus can update the setting in a synchronized fashion, based on the control information (refer, for example, to Japanese Laid-open Patent Publication No. 2003-115796, Published Japanese Translation of PCT application No. 2002-509381, and Japanese Laid-open Patent Publication No. 2002-185400).
In one such prior art example, when a base station controller generates or changes information concerning high speed forward packet connection service, the generated or changed information is transmitted to the base station and terminal equipment along with an indicator indicating that a change has been made to the information. The base station and the terminal equipment receive the generated or changed information and operate at the same time in an identical manner. According to this prior art, the timing at which the base station controller changes the information is defined by the system frame number (SFN), and its timing is reported from the base station controller to the base station and the terminal equipment.
In another prior art example, at the beginning of a handover, a radio network controller orders each serving base station to measure the transmit power to a mobile station being served. Each serving base station then measures its transmit power to the mobile station and reports the power level measurements to the radio network controller. An initial transmit power setting is determined for a new target base station being added in a diversity handover situation, and, for example, new transmit power settings are determined for the serving base stations synchronized to a particular time. At or about synchronizing time, the target base station transmits a signal to the mobile station at the initial power setting, and at or about the same time, the serving base stations adjust their transmit powers toward the new values.
In still another prior art example, each base station examines the amount of loss of a base station select signal and, if the amount of loss exceeds a given value, its transmit power is not reduced even if the base station is not a main base station. Each base station in a soft handover situation constantly updates the main base station conditions at a predetermined timing so that the main base station update timing at the mobile station is synchronized. Further, in this prior art example, the output conditions are updated at all base stations, based on the downlink slots of the same series number.
According to the above prior art, the timing at which the base station apparatus and the mobile station apparatus change the setup information is determined by the RNC. Then, each time the setup information is changed, the RNC notifies the base station apparatus and mobile station apparatus of the timing to change the setup information. Further, the mobile station apparatus and the base station apparatus each transmit a message to the RNC to report the reception of the notification. As a result, each time a change occurs in the setup information, the control information is transferred back and forth between the respective apparatuses.
Further, when the mobile station apparatus is communicating with some other apparatus, the number of communication cells to which the mobile station apparatus is linked constantly changes. Then, each time the number of communication cells changes, control information necessary to change the number of communication cells is transferred between the RNC, the base station apparatus, and the mobile station apparatus. The larger the number of mobile station apparatuses connected to the mobile communication system, the larger the amount of communication associated with the transfer of the control information becomes. As a result, as the number of mobile station apparatuses connected to the mobile communication system increases, the network load and the power consumption for the entire mobile communication system increase. In some cases, congestion may occur as the amount of communication increases, eventually bringing down the entire mobile communication system.